1. Field of the Invention
The present invention relates to an integrated magnetoresistive sensor and, more particularly, to an integrated magnetoresistive sensor used in rotational amount detection, position detection, and the like of an object.
2. Description of the Prior Art
In general, a magnetoresistive sensor is utilized in rotational amount detection by detecting a change in magnetic field in a flow rate meter of a hot water supply system, a speed meter of a vehicle, and the like, and is also utilized in position detection of a cylinder incorporated in a robot by a switching operation.
A conventional integrated magnetoresistive sensor is characterized by detection of a magnetic field which reciprocally moves in a predetermined direction, as described in, e.g., J. Appl. Phys. 69(8), Apr. 15, 1991.
FIG. 1 is an equivalent circuit diagram showing the arrangement of a conventional integrated magnetoresistive sensor. Referring to FIG. 1, a conventional magnetoresistive sensor includes a magnetoresistive element portion 1 consisting of four bridge connecting resistors 11 to 14, and a waveshaping processing portion 2 for waveshaping the output from the portion 1. Each resistor is form aligning a large number of magnetoresistive elements in an array, as shown in FIG. 2, and each magnetoresistive element consists of a ferromagnetic thin film (NiFe deposition film). A conductive film such as an Au film is formed on pattern returning portions (hatched portions) of the resistors 11 to 14.
In order to detect the direction of a magnetic field, the four resistors are arranged so that two adjacent resistors have orthogonal maximum detection directions, respectively. More specifically, when a magnetic field is applied in the direction of an arrow A, the resistances of the resistors 11 and 14 become maximum, and those of the resistors 12 and 13 become minimum. On the other hand, when a magnetic field is applied in the direction of an arrow B, the resistances of the resistors 12 and 13 become maximum, and those of the resistors 11 and 14 become minimum. In general, the direction of a magnetic field corresponding to a maximum resistance is called a direction of easy axis, and the direction of a magnetic field corresponding to a minimum resistance is called a direction of hard axis.
Assume that the resistors 11 to 14 have an electrical resistance of 10 k.OMEGA. or higher.
In the magnetoresistive element portion 1, a terminal 15 is connected to a power supply voltage Vcc, a terminal 16 is connected to ground level, a terminal 18 is connected to the plus input terminal (non-inverting input terminal) of a comparator, and a terminal 17 is connected to the minus input terminal (inverting input terminal) of the comparator. Referring to FIG. 1, the terminal 15 is connected to a power supply terminal 31, and the terminal 16 is connected to a ground terminal 33. The terminal 18 is connected to the plus input terminal (+) of a comparator 21 in the waveshaping processing portion 2, and the terminal 17 is connected to the minus input terminal (-) of the comparator 21.
The comparator 21 in the waveshaping processing portion 2 performs processing for calculating a potential difference between voltage values input to the plus and minus input terminals. The comparator 21 has two threshold levels. When the potential difference level exceeds the first threshold level, the comparator 21 outputs a low-level signal. On the other hand, when the potential difference level decreases below the second threshold level, the comparator 21 outputs a high-level signal. More specifically, the comparator 21 serves as a hysteresis comparator, and the potential difference between the two threshold levels is determined by a feedback resistor 22.
The feedback resistor 22 is normally connected to positively feed back the output from the comparator 21. In this arrangement, the feedback resistor 22 is connected between the plus input terminal of the comparator 21 and a terminal 34. This is to use the terminal 34 in an intermittent operation of this sensor so as to reduce power consumption when a battery is used as a power supply. More specifically, the previous output level is temporarily stored in an external memory (not shown), and the feedback amount to be supplied to the terminal 34 is changed in accordance with the stored level, thereby executing an operation continued from the previous operation. An access to the external memory (not shown) is made by a CPU (not shown). When the sensor is to be continuously operated, the feedback operation of the output from the comparator 21 to the plus input terminal thereof may be conducted. Note that the resistor 22 has a resistance of 1.5M .OMEGA. or higher.
An NPN transistor 24 is connected to the output terminal of the comparator 21, and its open collector serves as an output terminal 32. (Note that a set resistor 23 is connected between the comparator 21 and the ground terminal 33.)
The operation of the conventional integrated magnetoresistive sensor with the above-mentioned arrangement will be described below. Referring to FIG. 2, if the resistances of the resistors 11 to 14 are respectively represented by R1 to R4, and the angle defined between the direction of the applied magnetic field and the direction of the arrow B in FIG. 2 is represented by .theta., changes in resistance in a rotary magnetic field exceeding a saturated magnetic field are respectively given by: ##EQU1## where Rnmax (n=1 to 4) is the resistance obtained upon magnetization in the direction of easy axis, and Rnmin (n=1 to 4) is the resistance obtained upon magnetization in the direction of hard axis. In general, the sensor is designed on the basis of the resistance obtained upon magnetization in the direction of easy axis.
Output voltages VIN1 and VIN2 from the minus and plus terminals of the comparator 21 are respectively given by: EQU VIN1=Vcc*R3/(R1+R3) EQU VIN2=Vcc*R4/(R2+R4)
where Vcc is the power supply voltage value between the power supply terminal 31 and the ground terminal 33.
The output level from the output terminal 32 of the comparator 21 changes from high level to low level when a potential difference VIN (=VIN2-VIN1) between the minus and plus terminals exceeds a sum (first threshold level) of an offset voltage VIO and a hysteresis width VHYS provided to the comparator 21; and it changes from low level to high level when the potential difference VIN decreases below VIO (second threshold level).
If it is assumed that Rnmax (n=1 to 4)=R0, and the maximum change amount of the resistance upon application of a magnetic field is represented by .DELTA.R, a maximum change amount .DELTA.V of the VIN is given by: ##EQU2## Note that equation (1) above is approximately established.
FIG. 3 shows the detection characteristics obtained when a rotary magnetic field is applied to the conventional integrated magnetoresistive sensor described above. FIG. 3 shows the potential difference VIN between the minus and plus terminals of the comparator 21 when the response frequency=200 Hz.
Referring to FIG. 3, VTH1 corresponds to the first threshold level, i.e., VIO+VHYS, and VTH2 corresponds to the second threshold level, i.e., VIO. A rectangular wave which goes to low level when VIN exceeds VTH1 or goes to high level when VIN decreases below VTH2 is output from the comparator 21. Note that the duty ratio of the rectangular wave is 50% in the characteristics shown in FIG. 3.
In the conventional integrated magnetoresistive sensor with the above-mentioned characteristics, upon applying a power supply voltage to the sensor, the resistances of the resistors 11 to 14 fluctuate and the initial output level from the comparator 21 is not stabilized. In order to solve this problem, the above-mentioned VTH2 (or VTH1) may be set to be relatively high (or low) to stabilize the initial output level. However, in order to maintain a duty ratio of 50%, since the difference between VTH1 and VTH2 cannot be changed, both VTH1 and VTH2 must be set to be relatively high (or low). Then, the value VTH1 may become higher than the maximum value of the waveform of VIN (or the value VTH2 may become lower than the minimum value of the waveform of VIN). In this case, the value VIN becomes always lower than VTH1 (or higher than VTH2), and the output level from the comparator is fixed at high level (or low level). Therefore, it is difficult to fix the initial level.
In the above-mentioned integrated magnetoresistive sensor, the variation amount of output voltage .DELTA.V from the magneto-resistive element portion 1 is .DELTA.V=.DELTA..rho./.rho..times. Vcc when the power supply voltage is Vcc, and depends on only the ratio .DELTA..rho./.rho. of change in magnetoresistance. Since the ratio .DELTA..rho./.rho. of change in magnetoresistance of the NiFe deposition film which forms the magnetoresistive element portion 1 as a ferromagnetic thin film is about 2 to 3%, if the output voltage .DELTA.V is input to the comparator or the like without being amplified by, e.g., a preamplifier, the variation amount of the output voltage with respect to the detection level of the waveshaping processing portion 2 is undesirably small.